The present disclosure relates to wet friction engagement elements including annular friction plates each having a friction material bonded to its surface and annular separator plates capable of frictionally engaging with the friction plates.
Conventionally, wet multi-plate clutches including annular friction plates meshing with splines formed in the outer periphery of a hub and each having a friction material bonded thereto and annular separator plates meshing with splines formed in the inner periphery of a drum are known as this type of friction engagement element (see, e.g., Japanese Patent Application Publication No. 2000-145819). In this wet multi-plate clutch, a plurality of lubricant oil supply holes are formed in the hub. A plurality of oil through grooves extending from the inner peripheral edge to the outer peripheral edge of the friction material of the friction plate are formed such that their openings at the inner peripheral edge face lubricant oil supply holes. This allows the lubricant oil that is supplied from the lubricant oil supply holes throughout the spaces between each friction plate and each separator plate via the oil through grooves to flow more smoothly, and thus allows the lubricant oil to be discharged from the spaces between each friction plate and each separator plate in a more desirable manner.
As this type of friction engagement element, a wet multi-plate clutch is also known which includes a plurality of metal plates fitted in the inner periphery of an input drum and a plurality of facing plates fitted on the outer periphery of an input clutch hub and each having a friction material facing bonded to its both surfaces (see, e.g., Japanese Patent Application Publication No. 2002-106597). In this wet multi-plate clutch, oil grooves are formed in the surface of each metal plate so as to extend from the inner radial side toward the outer radial side, and clearance extending in the circumferential direction is provided between the inner periphery of the input drum and the outer radial ends of the oil grooves of each metal plate. Lubricant oil used to lubricate the metal plates and the facing plates thus flows in the axial direction of the wet multi-plate clutch from each oil groove to the spaces between the input drum and the plurality of metal plates through the clearance, and is discharged to the outside of the input drum through oil discharge openings formed in the input drum. A method in which a plurality of segments made of a friction material are bonded at intervals to a core plate having external teeth and a plurality of grooves are formed between the segments is also known as a method for manufacturing a wet friction clutch plate for use in this type of friction engagement element (see, e.g., Japanese Patent Application Publication No. 2004-176915).